1. Field of the Invention
The present invention relates to a multi-layered stretched resin film which is resistant to becoming waved or curled due to contact with the vehicle of offset printing inks during offset printing, and thus has an excellent printing properties. The multi-layered stretched resin film of the present invention may be used as a material for tracing paper, maps, book covers, wrapping paper, book paper, sticker sheets, poster paper, menu sheets, hanging screens, labels, shopping bags, cardboard, brochures, illuminated sign board and so forth.
2. Description of the Background
Synthetic paper has been proposed and put into practical use in recent years to replace conventional natural pulp-made paper. The synthetic paper comprises a base layer made of a biaxially stretched polypropylene film containing 5 to 40 wt % of an inorganic fine powder, and paper-like layers disposed on the top and bottom surfaces thereof, made of a uniaxially stretched polypropylene film containing 8 to 65 wt % of an inorganic fine powder (e.g., as described in U.S. Pat. No. 4,318,950, Japanese Examined Patent Publication Nos. 46-40794, 60-36173 and 62-35412).
Offset printing, (i.e., lithography), is generally used for printing on paper, polyethylene terephthalate film, polyamide film, coated paper and the like, since this technique can readily provide multi-color printing. The typical composition of a general-purpose, dry-type offset printing ink is shown in FIG. 1 and Table 1 below.
The vehicle shown in FIG. 1 refers to a liquid component, among the various components of which the printing ink is composed, which has the function of dispersing the pigment, thereby maintaining the fluidity of the ink; ensuring the smooth flow of the ink from the ink pot to a printing surface while the ink is transferred among individual rollers, the printing plate and printing blanket; and for fixing the pigment onto the printing surface by solidification of the vehicle after printing. In response to the recent strong requirement that the offset printing ink have a shortened drying time to allow more rapid printing, there is an increasing trend toward the used of quick-drying inks which are compounded with a vehicle which is mainly comprised of a drying oil mixed with resin and mineral oil (high-boiling-point, petroleum-base solvent).
However, synthetic paper comprising a stretched polyolefinic film or such film compounded with an inorganic fine powder is printed with such quick-drying offset printing inks, vehicle of the quick-drying offset printing ink swells the polyolefin of the synthetic paper, thereby producing local surface waving or curling of the printed film. This is a significant practical obstacle in using such synthetic papers. Conventional offset printing inks for polyolefinic film are therefore specially formulated to be free from mineral oil. However, this approach sacrifices the quick-drying property. In addition, such special offset printing inks for polyolefinic films require long drying times, and are used mainly in small specialty printing shops and manufacturers. There has therefore been a strong need for a new polyolefinic film on which the general-purpose offset printing ink of the oxidation polymerization type (drying oil type) may be used.
In other words, in the typical printing shop, offset printing is practiced on pulp-based paper such as wood free paper and coated paper using generally-commercialized, quick-drying ink. Printing on polyolefinic film or synthetic paper in such printing shops requires the temporary replacement of such ink with the special offset printing ink which is suitable for such non-absorbent material. However, replacement of the conventional ink for pulp-based paper is time- and labor-consuming. As a consequence, conventional print shops have resisted printing on polyolefinic film or synthetic paper, which has been one factor preventing the general use of polyolefinic film or synthetic paper in offset printing.
One approach to solving this problem has been suggested in Laid-Open Japanese Patent Publication No. 8-333466, in which a polyolefinic resin mixed with an amorphous resin is used as a part of the multi-layered film. However, this approach is still not completely successful in solving the problem of the swelling of the film (referred to as xe2x80x9csolvent attackxe2x80x9d hereinafter) due to the vehicle (in particular high-boiling-point, petroleum-based solvents such as mineral oil) in the offset printing ink.
It is therefore an object of the present invention to provide a multi-layered stretched resin film having excellent printing properties, which is less prone to surface waving or curling entirely, even if the film is printed with general-purpose, quick-drying offset printing ink.
The present inventors have found that adding an amorphous resin in a proper ratio to the surface layer of a thermoplastic resin film, and stretching this layer so as to substantially prevent voids from being formed, can successfully provide a multi-layered stretched resin film having excellent printing properties.
The present invention is a multi-layered stretched resin film which comprises a base layer (A) containing more than 90 wt % of a polyolefinic resin, and less than 10 wt % of an inorganic fine powder or an organic filler; and an amorphous resin-containing layer (B) provided on at least one side of the base layer (A), which contains 0 to 85 wt % of a polyolefinic resin and 15 to 100 wt % of an amorphous resin. In addition, the amorphous resin-containing layer (B) has a porosity of 5% or less. A particularly preferred multi-layered stretched resin film according to the present invention further comprises a surface layer (C) containing 30 to 85 wt % of a polyolefinic resin and 15 to 70 wt % of an inorganic fine powder or an organic filler. (In this specification, any numerical range indicated by values placed before and after the word xe2x80x9ctoxe2x80x9d defines a range inclusive of the minimum and maximum values.)